1. Field of the Invention
This invention relates to pacing leads having an anchoring mechanism such as tines and, more particularly, a lead having means for changing the peripheral geometry of the anchor mechanism to provide a reduced cross-section at times of introduction of the lead and/or repositioning of the lead.
2. Background of the Prior Art
Pacing leads using an anchoring mechanism for anchoring the distal tip with respect to the inside of the heart are well known and very important to a successful pacing system. The most successful and widely used anchoring mechanism is that of tines, as disclosed in U.S. Pat. No. 3,902,501, assigned to Medtronic, Inc. The tined lead provides a plurality of pliant tines that extend from an area adjacent the distal tip and electrode of the lead, the tines forming an acute angle with the lead body. The tines are effective in engaging the trabeculae found in the ventricle as well as the atrium, to maintain the electrode tip in a secure position after the physician has positioned it for good pacing threshold.
While the tined lead has been highly successful, it has carried one longstanding drawback, namely the problems associated with the lateral projections of the tines at the time of introduction of the lead and/or in attempting to reposition the lead. Basically, the lateral extension of the tines causes an effective increased diameter of the lead at about the distal tip, i.e., the effective diameter is much greater at the distal tip than it is along the length of the lead, which length has a suitable uniform small outer diameter. The tines can get stuck at the time of introduction, particularly in passing through the valve between the atrium and the ventricle for positioning against the inner wall of the right ventricle. Further, once the physician has positioned the distal tip against the heart wall, it may be desirable to change that position, in either acute or chronic situations, to obtain a position which offers improved pacing threshold; or to withdraw the lead entirely. In such a situation, withdrawal of the tined anchor mechanism may be difficult, or even impossible, to do without damage to the trabeculae or to a valve.
The difficulty of introducing a pacing lead with a tined anchoring element was recognized in the first patent on the tined lead, the above-noted Citron et al. U.S. Pat. No. 3,902,501. That patent showed an embodiment which included a mechanism for holding the tines against the electrode body during insertion, while allowing their release once the tip was positioned in proximity to the heart wall. However, the mechanism disclosed has not been effective in achieving the aim of reducing the tip cross-section while reliably providing for release of the tines after insertion of the lead. Further, this mechanism did not have any capability of transforming the tines into a reduced cross-sectional geometry which would permit easier withdrawal of the anchor when and as desired. What remained necessary at that time, and still remains a need in the art, is a design which provides both for avoiding the lateral extension of the tines or other anchor mechanism during introduction, and also one which can withdraw the tines in such a manner as to provide improved characteristics for withdrawing the lead after it has already been positioned against the heart wall.
Since the introduction and commercial success of the tined lead, there have been a number of efforts in the pacing industry, and disclosed in the patent literature, to provide an anchoring mechanism which would provide improved characteristics for minimizing the above-noted problems at introduction and/or repositioning. German Application 33 00 050 discloses an intravenous heart pacemaker electrode having a fixation or anchor means that can be shifted along the longitudinal axis of the lead in a manner so as to reduce the effective diameter of the anchor mechanism. However, this design is applicable only for a lobe-type anchor element, and it does not suggest a solution to the problem with respect to tines which extend laterally to a free end. Another patent disclosing an extensible passive fixation mechanism for a pacing lead is U.S. Pat. No. 4,913,164. This patent shows a mechanism for moving tine-type elements from a first unextended position to a second extended position, or vice versa. However, the mechanism is complex, and the technique of closing the tine-type elements would tend to ensnare the lead tip in prior-engaged trabeculae, rather than provide easy withdrawal. The Earlebacher U.S. Pat. No. 4,957,118 shows another form of electrode lead having a tine assembly, wherein the tines can be actively moved back and forth between a retracted position and an extended position. However, the mechanism of this disclosure likewise results in a similar problem for extraction, namely the tine element is brought down over or on top of the engaging trabeculae. Further, when in the closed position, the tine end is free and provides a space between the tube casing and the tine which would snag trabeculae when the physician attempts to withdraw the lead.
There remains a need in the art for a pacing lead which provides an effective anchoring mechanism when the lead is in position, but having means to reduce the effect of such anchoring mechanism when desired, such as during introduction or repositioning of the lead. Stated differently, there is a need for reducing the effective cross-section of an anchor mechanism such as found in a tined pacing lead, whereby the anchor mechanism is transformed to a geometry which as closely as possible merges with the cylindrical lead casing, and which does not obstruct withdrawal of the lead when in the transformed state.